1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof, and more particularly to, a transfer device employed in the image forming apparatus.
2. Description of the Related Art
Known image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photosensitive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
An electrostatic transfer method is widely employed in image forming apparatuses to transfer the toner image formed on the image bearing member onto an intermediate transfer member or directly onto a recording medium. In such electrostatic transfer method, a transfer bias having a polarity opposite that of the toner is supplied to form a transfer electric field which enables toner to move from an image bearing member to the recording medium or to the intermediate transfer member.
The above-described transfer method can be applied to single-color image formation in which a toner image is transferred from a single image bearing member and to multiple-color image formation in which a plurality of toner images are transferred to the intermediate transfer member one atop the other to form a composite toner image, and the composite toner image is then transferred onto a recording medium.
A variety of recording media sheets such as textured or embossed paper having a rough surface have come on the market. A recording medium having a relatively large embossed groove depth or having a low smoothness has a rough surface. When transferring a toner image onto the rough surface of the recording medium, there is a gap between the image bearing member and a recessed portion of the surface of the recording medium, and the size of the gap varies between the recessed portions. As a result, the toner is not transferred sufficiently onto the recessed portions, causing low transferability of toner and hence resulting in image defects such as dropouts and blank spots.
In order to prevent improper transfer of toner, JP-2000-19854-A proposes to detect the gap formed between the recessed portions of the recording medium and the image bearing member at a transfer nip where the toner image is transferred onto the recording medium. After detecting the gap, an oscillating electric field is applied to the transfer nip, thereby moving the toner back and forth in the transfer nip. In this configuration, a frequency of toner contacting the recessed portions is increased, hence increasing transferability of toner.
When forming the oscillating electric field, an absolute value of a peak voltage exceeds a voltage with a steady electric field. As a result, leakage occurs at places such as an end portion of a transfer device (for example, a transfer roller) constituting the transfer nip, an insulating member that insulates from a housing an electrode to which a high voltage is applied, inside a high voltage power source for outputting a high voltage, between the housing and a high-voltage power line and a connector connecting the high voltage power source and the electrode. The leakage increases power consumption, hence increasing the running cost of the image forming apparatus. Moreover, the leakage adversely affects other electronic devices.
In view of the above, there is thus an unsolved need for an image forming apparatus capable of suppressing leakage when transferring toner using an oscillating electric field.